The third class mentioned, the charcoal of wood, is capable of affording an intense heat. Mr. Dalton, by heating water, obtained a result equivalent to melting 40 lbs. of ice with 1 lb. of charcoal. Dr. Crawford's experiments give 69 lbs. of ice melted by 1 lb. of charcoal. Lavoisier, Clement, and Desormes, about 95 lbs.; and Hassenfratz, 92 lbs. Mr. Tredgold considers 47 lbs. of ice melted to be the real average effect of 1 lb. of charcoal : a cubic foot weighs about 15 lbs.
The fourth mentioned class of fuel, pit coal charred or coke, possesses similar properties to wood charcoal, although it is a much stronger fuel,—that is, it contains the combustible matter in a more condensed form ; it is, there fore, consumed much more slowly, and is better adapted for long-continued intense heats. It has, however, a defect, from which wood charcoal is free ; it leaves dense ashes in the grate, which in time collect in such quantity as to obstruct the passage of the air ; and when the heat is intense, these ashes vitrify into a tenacious substance, which clogs the furnace. It is preferable to wood coal for melting metals, as affording a greater quantity of heat before it is consumed, and at a less expense.
The fifth class of fuel, according to Dr. Black, is wood and crude coal; these differ from their charcoals in affording copious and bright flames when plenty of air is admitted to them. If but little air be admitted sooty vapours are given out without flame, and with greatly diminished heat. Wood and candle coal do, however, differ from each other so much, as respects their useful pro perties in manufacturing operations, that we deem it necessary here to drop the generalization of Dr. Black, and consider wood and coal, and the varieties of each, separately. First, as respects— Wood : its effect in producing heat depends greatly on its state of dryness. Several experiments made by Count Rumford show the effect of dry wood to be much greater than that of unseasoned ; the latter containing about one-third of its weight of water. The kind of wood is also a cause of some difference ; lime-tree wood was found, by Count Rumford, to give out most heat in burn ing. With 1 lb. of dry pine-wood, the Count caused 20.10 lbs. of ice-cold water to boil. The same weight of dry beech made only 14.33 lbs. of ice-cold water to boil. A cubic foot of dry beech weighs about 49 lbs. By the experi ments of Fossombroni, wood was found capable, by its combustion, to evapo rate twice its weight Of water, or to prepare two-thirds of its weight of salt. Rumford made the effect about one-third more than Fossombroni, owing, pos sibly, to superior management in the former. By an experiment made at the Opera House, in Paris, 160 lbs. of wood were found to be equal in effect to 58 lbs. of coke.
As respects coal, there is a considerable difference in the effects of the several varieties. The caking or binding coal with which London is supplied from the great coal fields in Northumberland and Durham, under the general name of Newcastle coal, is much esteemed, from its affording a great heat, and burning with a lively flame ; but those of Wall's End are regarded as superior to the latter for domestic use, as they burn with a whiter and more brilliant flame, and do not cake so hard in the grate. The Tanfield Moor coals are preferred for
forges and furnaces, as they burn slowly, and afford a strong and long continued heat. From the experiments of Mr. Watt, it appears that a bushel of New castle coal, which weighs about 84 lbs. is competent to convert from 8 to 12 cubic feet of water into steam, from the mean temperature of the atmosphere ; and that a bushel of Swansea coal will produce the same effect. Dr. Black states, 7.91 lbs. of the best Newcastle coals will convert one cubic foot of water into steam capable of supporting the mean pressure of the atmosphere ; and this statement appears perfectly to accord with the more extended experiments of Watt. Smeaton makes it require 11.4 lbs. of coal to produce the same result in steam ; but Smeaton has omitted to state the kind of coal. If he employed the Staffordshire coal, there is no discrepancy, as will appear from the table of Mr. Tredgold's experiments and calculations, which we shall subse quently insert in this article. Mr. Tredgold found, that after the brickwork, &c. of the boiler of a steam-engine was warmed, a little less than 1 lb. of Wall's End coals would make a cubic foot of water boil, from the mean temperature of 520. To produce the same effect with inferior coals a stronger draft and more time and attention arc necessary. Splint coal, or hard coal, called by Kirwan slaty cannel coal, is regarded as equally valuable for many purposes as the Newcastle caking coal. It does not produce so much flame nor so much smoke ; it does not kindle so quickly, nor does it agglutinate, like caking coal. A large body of splint coal makes a strong and lasting fire. Cherry coal, or soft coal, readily catches fire, and burns with a clear yellow flame, giving out much heat, and the flame continues till nearly the whole coal is consumed. It burns away more rapidly than either caking or splint coal, and leaves a white ash ; it is easily distinguished from caking coal by its not melting or becoming soft when heated; it makes a more agreeable fire, and does not require to be stirred. It requires care and management in an open grate, even to burn the small fragments which are made in breaking up the pieces to a fit size for the fire : hence the small coals are often mixed with clay, and made into balls. When these balls are dry, Mr. Gray says, they make an excellent addition to the fuel for an open fire, producing a very durable heat. Mr. Watt calculated that 112 lbs. of these coals produced the same effect in raising steam as 84 lbs. of the Newcastle coal.